The present invention relates to a nuclear fuel element for use in a nuclear reactor, preferably of the fast neutron type.
The nuclear fuel elements used in fast neutron reactors are generally in the form of rods, which are then brought together in bundles or groups in nuclear fuel assemblies constituting the fissile part of the reactor core.
Each nuclear fuel rod essentially comprises a tubular metal can or jacket, in which are stacked fissile material pellets constituting the actual fuel. The can is sealed at each of its ends by a plug, which is forced into the interior of the can and then welded level with the junction plane between plug and can.
Due to the tolerances inherent in the manufacture of tubular cans, the internal diameter thereof can vary by approximately 1/100 mm within the same production batch and by approximately 6/100 mm in different production batches.
To take account of these tolerances, the presently used plug fitting process consists firstly of pairing the plugs as a function of the internal diameter of the cans. When this pairing has taken place, the plugs are fitted into the cans with a force of approximately 80 daN for an average tightening of 2/100 mm, prior to being welding.
However, despite the precautions taken, there are still mixtures of cans obtained from different batches or non-conforming parts, which often makes it necessary for manufacturers to manually complete the plugging operation using a hammer and the elimination of plugs which do not hold in the cans. This leads to a very low working speed and to considerable wastage at the production stage.
Moreover, the approximate nature of the tightening between the plugs and the can often leads to excessive tightening, which can cause an abnormal swelling of the can and to a pulling away of metal, which often leads to defects in the welded area. This is another cause of wastage when these are latent defects. Among these defects, reference can be made to the plastic deformation of the bearing face during the stopping of the upper plug leading to a flattening which is prejudicial to the fitting of the lower plug and to the quality of its weld. A deformation at the weld of the lower plug also causes problems for fitting the rod bundles within the assemblies. Moreover, excessive tightening of the plugs in the can can lead to buckling of the latter, bearing in mind the considerable difference between its diameter and its length.
A consequence of all these difficulties is that it is not possible with the present technology to envisage an automation of the production of nuclear fuel elements.